Sample atomizing device having a radiation absorbing protective jacket for flameless atomic absorption spectroscopy

ABSTRACT

A known atomizing device, especially useful for atomizing the sample in a flameless atomic absorption spectrometer, includes a hollow body accommodating the sample which is heated electrically to a high temperature; this body is typically a graphite tubular sample cell which is heated up by applying a large current to its opposite ends. It is proposed to surround this body by at least one radiation absorbing (and therefore re-emitting) protective jacket, for example, a tubular member also made of graphite. This protective jacket reduces the amount of electric energy necessary to heat the body to a particular temperature since a substantial part of the radiation emitted from the body is absorbed and reemitted back to the body by the protective jacket. In an alternative construction the protective jacket comprises a relatively deep layer of a porous material such as porous coal, so that the non-conducting pores cause the jacket to act substantially like a plurality of separate portions of concentric jackets.

United States Patent a 8 Braun et al.

[111 3,819,279 [4 June 25, 1974 [73] Assignee: Bodenseewerk Perkin-Elmer& Co.,

GmbH, Bodensee, Germany [22] Filed: Apr. 20, 1973 [21] Appl. No.:353,088

[30] Foreign Application Priority Data.

Apr.2l,l972 Germany 2219594 52 US. Cl. 356/244, 356/85" [51] Int. ClG0ln 21/16, GOlj 3/30 [58] Field of Search 356/85, 87, 244

[56] References Cited UNITED STATES PATENTS 6/1972 Wiedeking 356/24411/1972 Braun et al. 356/244 Primary ExaminerR0nald L. Wibert AssistantExaminer-V. P. McGraw Attorney, Agent, or Firm-Daniel R. Levinson [5 7]ABSTRACT A known atomizing device, especially useful for atomizing thesample in a flameless atomic absorption spectrometer, includes a hollowbody accommodating the sample which is heated electrically to a hightemperature; this body is typically a graphite tubular sample cell whichis heated up by applying a large current to its opposite ends. It isproposed to surround this body by at least one radiation absorbing (andtherefore reemitting) protective jacket, for example, a tubular memberalso made of graphite. This protective jacket reduces the amount ofelectric energy necessary to heat the body to a particular temperaturesince a substantial part of the radiation emitted from the body isabsorbed and re-emitted back to the body by the protective jacket. In analternative construction the protective jacket comprises a relativelydeep layer of a porous material such as porous coal, so that thenonconducting pores cause the jacket to act substantially like aplurality of separate portions of concentric jack- MS.

7 Claims, 2 Drawing Figures is maintained.

, 1 SAMPLE ATOMIZING DEVICE HAVING A RADIATION ABSORBING PROTECTIVEJACKET FOR FLAMELESS ATOMIC ABSORPTION SPECTROSCOPY This inventionrelates to a device for atomizing a sample for flameless atomicabsorption spectroscopy, in which a body accommodating the sample beingatomized'can be heated electrically to a high temperature. Such a deviceis, for instance, already known in the form of a graphite tube which isheated to glow temperature by passage of electric current, a shortcomingbeing the high radiation losses particularly at high atomizingtemperatures which require a correspondingly high electric power input.Especially at high temperatures, the radiation losses increase veryquickly with increasing temperature so that with a considerable increasein the power only a relatively small increase in temperature can beachieved.

Moreover, it is already prior art to reflect at least partly theradiated power back again onto the graphite tube. For this purpose, thechamber containing the graphite tube is designed to be specularlyreflecting (i.e. metallic) or diffusely reflecting. The reflection,however, is not perfect. As compared with blackened chambers, at themost a temperature increase by about 100 C can be achieved by thisreflecting technique at high atomizing temperatures. Besides, thechamber very quickly blackens during use, as the highly heated graphitetube emits graphite particles which deposit on the chamber walls.

It is an object of this invention to improve the relationship betweenvthe achieved temperature of the atomizing device and power spent toreach this temperature.

According to the invention this object is attained by providing that thesurfaces of the atomizing body turned away from the sample, andtherefore away from the resulting atomic cloud, are surrounded by atleast one radiation-absorbing protective jacket having low heatconductivity.

For elucidation of the inventive idea it shall be assumed that a glowinggraphite tube of the temperature T is disposed in a chamber whoseinternal walls are blackened and are maintained at the temperature T bycooling water. Then, the radiation losses S are S K 3 o) 1 wherein K isa constant which, inter alia, includes the effect of particularradiating surface of the graphite tube.

Now, let a protective jacket of graphite be slipped over the graphitetube concentrically therewith. The protective jacket absorbs theimpinging radiation, is thereby heated itself, emits part of the heatenergy in the form of radiation back onto the graphite tube, whileemitting another part outwardly towards the chamber walls. Thetemperature of the protective jacket T, will become balanced. Then, fromthe graphite tube to the protective jacket a radiation flux of themagnitude Between the protective tube and the chamber walls aradiationflux of the'magnitude is obtained. v

The radiating surface of the protective jacket is greater onlymarginally than that of the graphite tube, so that without substantialerrors K 1 K. When furthermore neglecting the heat conduction throughthe protective tube, then in a state of equilibrium, both radiationfluxes must be equally great:

wherefrom it can be concluded:

T14 T To 5 When substituting this value in equation (2), then for theradiation of the graphite tube S it is found:

i K 11 0 r wherein the subscript l characterizes the use of a singleprotective tube. Thus the radiation losses of the graphite tube arereduced to half.

If more than one protective tube are arranged at spaced distancesconcentrically with respect to each other, then the radiation losseswill be reduced even more. Here, too, when neglecting the heatconduction through the protective tube, the radiation fluxes from eachone of the protective tubes to the next one must be equal to each other.When numbering the protective tubes from the outside inwardly, thus,when referencing the temperature of the outer protective tube T the nextinner one T and so on, then the following applies:

Tn T, T T T T04 7 The radiation flux without protective tube isproportional to T, T of those with n protective tubes proportional to TT Simply stated mathematically, it is found that When-considering theequation (7), it results:

R" o"=( nn) 9 or, if S, represents the radiation losses with nprotective tubes and S the radiation losses without any protectivetubes:

Thus, by the use of several protective tubes, the radiation losses canbe reduced quite substantially. However, it would be unwise to selectthe nember of protective tubes excessively great, for on the one hand,in this manner also the mass being heated is increased, which has adisadvantageous effect on the rate of temperature rise, while, on theother hand, by heat conduction in the inner protective tubes a certaintemperature differ- 3. ence is already maintained which cannot bepractically reduced further by additional protective tubes fartheroutwardly.

The radiation losses of the atomizing device, for instance, of thegraphite tube, are reduced in that opposite to the outwardly radiatingsurface of this atomizing device another absorbing surface is arrangedwhose temperature is only slightly below the temperature of theatomizing device and which returns part of the absorbed radiation backonto the atomizing device as by emission. The greater the number ofprotective jackets is, the less is the temperature difference betweenthe atomizing device and the first protective jacket surface, thus theless are the radiation losses.

The device according to the invention is distinguished from the previousattempts of improving the relationship of temperature and electricpower, in that the heat which is radiated by the graphite tube is notreflected back onto the graphite tube by the cooled housing. Rather theprotective jacket absorbs the radiated heat and becomes itself heated,so that it emits thermal radiation towards the grahpite tube or thelike.

Experimentally it has shown that the maximum temperature of a graphitetube can be increased by 300 C at the same electric power by the use ofa tubular protective jacket surrounding the graphite tube.

At least one solid protective jacket can surround the body at a spaceddistance therefrom. In a further modiflcation, the protective jacket canconsist of a porous material such as porous coal. Such a porousprotective jacket corresponds in its effect to a plurality of protectivejackets.

Heat conduction is strongly reduced by the porous structure. A greatpart of the heat transmission is effected internally of the pores bythermal radiation. But the heat transfer by radiation is very smallsince internally of each pore only small temperature differences occur.In order to reduce the heat radiation of the protective jacketoutwardly, the protective jacket can have its outside renderedreflecting.

Two illustrative embodiments of this invention will now be describedmore fully with reference to the accompanying drawing in which:

FIG. 1 is a longitudinal section through a graphite tube cell having aprotective jacket of graphite.

FIG. 2 is a longitudinal section through a graphite tube cell having aporous protective jacket.

FIG. 1 illustrates schematically a graphite tube cell in longitudinalsection. To the graphite tube 1 the electric current is supplied viacontact cones 2 of graphite which are mounted in cooling chambers 3. Theleft contact cone has an annular recess 4 into which the protective tube5 is inserted, which is also made of graphite. To allow sampleintroduction, the protective tube 5 is provided with a lateral bore 6which is disposed above the corresponding bore 7 of the graphite tube 1.

A modification of this embodiment consists in that the protective tube 5is not made of graphite, but of a reflecting material, such as metal.Then, the protective tube will blacken on the inside by emitted graphiteparticles during use, but will remain bright (reflecting) on theoutside. Thereby, radiation of the protective tube outwardly will bereduced. The temperature of the protective tube rises, and the powerradiated towards the graphite tube increases so that the resultantradiation transmission from the graphite tube to the protective tube isreduced. Here too, the essential inventive idea consists in that theradiation emitted by the graphite tube is not reflected back, but thatthe protective jacket is heated so strongly by radiation absorption thatas much radiation as possible is emitted inwardly and is again absorbedby the graphite atomizing tube.

FIG. 2 illustrates another embodiment. Herein, the protective tube 5 isreplaced by a hollow-cylindrical body 10 of porous coal. The body 10 hasa lateral opening 11 for sample introduction and for the supply ofprotective (i.e., inert) gas. To avoid an electric shortcircuit, thereis provided an insulating layer 12 on the right side on the outside, andalso an annular insulating body 13.

Between the porous coal body 10 and the graphite tube 1 a narrow gasspace 14 is provided. If the porous body 10 is made so loose that only anegligibly small electric conduction occurs, then the coal body 10 mayalso be slipped directly over the graphite tube 1, i.e. in contact withthe same. This also applies to the protective jacket 5, if the sameconsists of an electrically insulating material.

Although the invention has been elucidated relative to two graphite tubecells, however, the measures according to the invention are alsoapplicable to other devices for atomizing a sample or the like.

We claim:

1. In a device for atomizing a sample for flameless atomic absorptionspectroscopy, in which a body accommodating the sample being atomized isheated electrically to a high temperature, the improvement comprising:

the surfaces of the body (1) facing away from the sample, and thereforeultimately the atomic cloud, are surrounded by at least one radiationabsorbing protective jacket (5;l0) having low heat conduction.

2. A device as claimed in the claim 1, in which:

at least one solid protective jacket (5) surrounds the body (1) at aspaced distance therefrom.

3. A device as claimed in the claim 1, in which:

said protective jacket (10) consists of a porous material, such asporous coal.

4. A device as claimed in the claim 1, in which:

said protective jacket (5) is designed to be reflecting on the outside.

5. A device as claimed in the claim 1, in which:

said body is a graphite tube (1) which has a contact cone (2) at eachend and is mounted with these contact cones (2) in correspondinglyconical surfaces of housing portions (3) through which coolant isflowing and through which the heating current is supplied, and whichsurround the graphite tube (1) like a jacket from both sides,

' and said tubular protective jacket (5) surrounds the graphite tube (1)and is arranged between the same and the housing portions (3).

6. A device as claimed in the claim 5, in which:

said protective jacket is a solid tube (5) which surrounds the graphitetube at a spaced distance therefrom said is mounted in a contact cone(2) on one side.

7. A device as claimed in the claim 5, in which:

said protective jacket is a thick porous tube as compared with thegraphite tube (10) and surrounds the graphite tube at a spaced distancetherefrom and is mounted in one of the housing portions (2) throughwhich a coolant is flowing, while being insulated relative to the otherhousing portion by an insulation (12; 13).

1. In a device for atomizing a sample for flameless atomic absorption spectroscopy, in which a body accommodating the sample being atomized is heated electrically to a high temperature, the improvement comprising: the surfaces of the body (1) facing away from the sample, and therefore ultimately the atomic cloud, are surrounded by at least one radiation absorbing protective jacket (5;10) having low heat conduction.
 2. A device as claimed in the claim 1, in which: at least one solid protective jacket (5) surrounds the body (1) at a spaced distance therefrom.
 3. A device as claimed in the claim 1, in which: said protective jacket (10) consists of a porous material, such as porous coal.
 4. A device as claimed in the claim 1, in which: said protective jacket (5) is designed to be reflecting on the outside.
 5. A device as claimed in the claim 1, in which: said body is a graphite tube (1) which has a contact cone (2) at each end and is mounted with these contact cones (2) in correspondingly conical surfaces of housing portions (3) through which coolant is flowing and through which the heating current is supplied, and which surround the graphite tube (1) like a jacket from both sides, and said tubular protective jacket (5) surrounds the graphite tube (1) and is arranged between the same and the housing portions (3).
 6. A device as claimed in the claim 5, in which: said protective jacket is a solid tube (5) which surrounds the graphite tube at a spaced distance therefrom said is mounted in a contact cone (2) on one side.
 7. A device as claimed in the claim 5, in which: said protective jacket is a thick porous tube as compared with the graphite tube (10) and surrounds the graphite tube at a spaced distance therefrom and is mounted in one of the housing portions (2) through which a coolant is flowing, while being insulated relative to the other housing portion by an insulation (12; 13). 